Hot work die block



June. 29, 1937. succop 2,085,676

-HOT WORK DIE BLOCK Filed Aug. 26. 1936 fiecalesence Volume Changes in Mills per inch.

I WITNESSES INVENTOR.

BY 1 4 4. 4%,, 2 50% v 4.1%, @3044 5M 4' ATTORNEYS.

Patented June 29, 1937 nor worn: ms mock John A. Succop, Pittsburgh, Pa., assignor to Heppenstall Company, Pittsburgh, Pa., a corporation of Pennsylvania.

Application August 26, 193 Serial No. 97,926 i 4 Claims.

This invention relates to articles of manufacture used in hot forming operations, more particularly to hot work die blocks.

Die blocks, which may be taken for purposes 5 of discussion as representative of the hot work articles contemplated by the present invention, are subjected to an unusual combination of stresses and destructive service conditions, these being more diverse and more severe than are commonly encountered in other commercial applications of steel. Similarly, apart from die blocks there are few uses" of steel wherein each production step, from melting through fabrication, heat treatment, and machining, plays such an important part in the provision of satisfactory service results. In most structural uses of steel the stresses while not entirely simple in themselves, are usually of a relatively simple nature and more or less predominantly of one type, such as straight tension or compression, or perhaps tension or compression with some subsidiary type of stress, such as shear or torsion. Again, die blocks are used at elevated temperatures, and at such temperatures the ability of 25 steels ,to resist stresses is generally less than at room temperature.

Accordingly, the physical requirements of but work die blocks are quite complex, due not only to the fact that they are exposed to very severe service conditions, but also to the fabricating problems, for if the die block successfully fulfills service requirements it is not unusual to en-'- counter difliculties in its production. However, to be commercially satisfactory and successful hot work die blocks must befully adaptable both to fabrication and service.

Chromium-nickelemolybdenum steels have been widely used for making die blocks, and their characteristics commonly are used as the .basis of commercial comparison of other die blocks and die block steels. Extended experience in the use of such die blocks has shown, however, that they are not wholly free from certain disadvantages which militate against them and are particularly objectionable in the manufacture and use of die blocks, and these may be referred to in further exemplification of the difficulties encountered in this field.

' Forinstance, during fabrication the chromiumforefor the-production of die blocks tend to develop the so-called directional structures. This type of structure results from segregation occursubsequent working causes the non-uniformities ing nickel.

nickel-molybdenum combinations used hereto-q Chromium-nickel-molybdenum steel.

ring during freezing of the molten steel, and.

of the structure to assume a directional orientation called directional structure.

While such directional structure is not desirable in any steel which is subjected to stress, nevertheless its presence is not as harmful in ordinarystructuril steels as it is in die steels. Steps are usually taken in the production of forging dies to overcome the effect of these structures by working all six sidesof the ingot. However, certain compositions tend more than others to develop these directional structures, which are readily revealed under the microscope as ghost lines or bandings. Their presence is deleterious not only because of the weakening effect on the general strength of the die block, due to the structural non-uniformity, but also because of the presence of split transformations which result from the same source and;which engender difficulties in processing and hardening by setting up, during the heat cycles involved, strains that manifest themselves asinternal cracks. These 20 disadvantages appear to inhere in steels contain- The-chromium-nickel-molybdenum steels used for die blocks likewise are strongly air-hardening. This is not desirable since the chilling of metal in an ingot mold may cause production of severe internal strains which cause internal ruptures that may not be healed in subsequent working, or which may subsequently cause untimely failure of the die. Similarly, cooling of a die block from forging temperatures may also cause the formation of microscopic or larger ruptures which lead to premature failure of the die block. That the chromium-nickel-molybdenum steels are particularly sensitive to such fabrication and service hazards appears from the literature on the subject. Thus in an article in Usine, Vol. 44, pages 27-29 (November 14, 1935) entitled Cracks in forged pieces it is stated that the susceptibility of steels to internal strain and rupture from this cause increases in the followmg order:

Carbon steel I Manganese steel (1 to 1.5% Mn) Manganese-silicon steel Chrome steel (1 to 2% Cr), with or without Mo- Nickel steel (1.5% Ni) Chromium-nickel steel A further characteristic of the chromiumnickel-molybdenum steels which I believe to be disadvantageous from the standpoint of die block "manufacture is that they exhibit markedly depressed recalescence ranges. The reason why cating problems.

' become too soft for satisfactory service.

cur, and if these take place at relatively verylow temperatures, ,where the plasticity of the steel is slight as contrasted with higher temperatures, the mass is less capable of flowing or adjusting itself internally to the high stresses which may be set up as a result of the structural changes. Considering the die block as a whole I believe that it is advantageous, therefore, to

have such structural changes take place on recalescence at higher rather than at lower, or depressed, temperatures.

Hardening depends upon the time rate of passing through the critical range. The rate of heat extraction is faster at elevated temperatures than at lower ones, and hence one does not have to resort to severe or rapid hardening procedures, or to air-hardening combinations, to produce satisfactory properties. Likewise, the progressive structural changes from the outside to the center of the block are confined, on cooling, to narrow limits when the critical temperatures are not strongly depressed. Thus, avoidance of marked depressed recalescence ranges renders the fabr'ication of the die block less hazardous.

In addition to the desirability of avoiding both segregation with consequent formation of directional structures, and also air-hardening, a satisfactory die block should not only be capable of responding throughout its mass to heat treatment with production of substantially uniform hardness and other properties throughout, but it should besusceptible to heat treatment bysimple and non-=hazardous procedures, and it should be capable of being economically machined or tooled in a condition ready for service, or at least; be capable of being heat treated after machining or tooling without warping, distortion, or formation of internal ruptures or cracks.

- The foregoing characteristics relate to fabri- The criteria of die block service and operation are likewise high and diflicult to satisfy, as well as being complex in themselves. In the first place, the die must undergo a minimum of dimensional changes and show a minimum of creep when subjected to the repeated rapid heating and cooling encountered in hot forming operations. Deformations in use result partly from strains due to heating, and partly from structural changes. The hardness and impact strength must be maintained under the "hot working conditions, and the drawing effectof the service heat must be negligible else the die may Similarly, the metal should not flow under the severe impact blows, even at elevated service temperatures which are built up or accumulated by repeated contacts with the metal being forged. The die block must possess a combination of tensile, compression and shear strength sufficient to prevent breakage and'deformation, together with ductility suflicient to prevent spalling and cracking. Again, the die block must resist the development of thermal cracks in service because when tliese start failure of the die block is im- 1 pending, and since hot forging dies are expensive it is to the inter'est of the user to obtain the greatest possible life from the die. To the same end the die must be wear resistant against the flow of the hot plastic metal being worked, as well] as against the highly abrasive action of sea e.

The foregoing are some of the service condifor use as a die block.

It is among the objects of this invention to provide die blocks of simple composition and of low alloy content, which may be fabricated satisfactorily, maybe oil-hardened without danger and by simple procedures to give satisfactory and substantially uniform hardness throughout the block, in which lowering of the recalescene points is not marked, which satisfactorily fulfill service requirements, and which afford improved operating and service characteristics as contrasted with the die blocks now available.

In accordance with the invention hot work die blocks are made from steel comprising about 0.4 to about 0.7 per cent of carbon, about 0.6 to 1.25 per cent of chromium, about 0.6 to 1.0 per cent of manganese, about 0.3 to 0.8 per cent of molybdenum, and about 0.08 to 0.2 per cent of vanadium. The balance of the steel comprises iron together with the impurities and elements normally occurring in the manufacture of such a steel, but for most purposes the silicon should be from about 0.2 to 0.4 per cent. It will be recognized that elements other than those specifically named may be present provided they do not adversely affect the desirable properties which characterize the steel for the manufacture of die blocks, and since from that standpoint such additional elements bear no function in respect of the gist of the invention they may be considered effectively as being iron.

For the majority of service conditions I prefer to make the die blocks from steel containing about 0.47 to 0.58 per cent of carbon, about 0.85 to 1.15 per cent of chromium, about 0.6 to 1.0 per cent of manganese, about 0.3 to 0.6 per cent of molybdenum, and about 0.12 to 0.15 per cent of vanadium. The silicon content is that normal to such a steel. 1

I have found that die blocks formed from steel of the composition given successfully fulfill the various fabricating, heat treatment and service requirements of a die block, including those delineated hereinabove, and that they are capable of providing much more satisfactory service life than the best die steels now in commercial use. For instance, these die blocks are characterized by reduction in or avoidance of directional structures heretofore encountered in some die block steels.

A major feature of the invention is that the die blocks show little tendency, or are less likely to undergo, the formation of internal cracks and ruptures in their production and use. This is of major importance in this particular field because from a given heat of steel 2. higher proportion of satisfactory die blocks is obtained, and the die blocks exhibit longer life, than where they are portance is the fact that the blocks reveal unusual stability after being drawn at elevated temperatures.

As exemplifying this aspect of the invention a' 5 forged die block made in accordance with the invention and 14 inches by 12 inches by 18 inches in size was normalized at 1600 F., hardened in ,oil from 1550" F., and drawn for twelve hours at 1100 F. After removing one-quarter inch from the surface the heat treated block showed a Brinell hardness of 341. It was redrawn for nine hours at 1100 F. and the hardness was then 2163 Brinell. After another ten hours at 1100 F. the hardness was 341. This property is extremely desirable in a hot work die block For most purposes it is preferred to heat treat the dies by normalizing the forged blocks at a temperature of,1600 F., followed by quenching in oil from 1550" F., and then drawing at 1000 to 1250 F., or as desired to produce the requisite hardness. My tests have shown that an average size die block so treated and drawn at 1000" F. possesses a hardness of about 55 to 60 scleroscope,

or a hardness of about 40 when drawn at 1 250 F. 5 The die blocks provided by the invention also exhibit commercially satisfactory uniformity of.

. hardness throughout their mass, which is of real importance in this art. A block of the size just referred to was heat-treated and then cold sawed longitudinally through the center, and hardness determinations .were made on 'two diagonals across the longitudinal cross-section, the results being as follows; a

In line with what was saidabove, these die blocks also exhibit but slightly depressed recalescence ranges. It has been determined through dilatometer tests of die blocks made in accordance with this invention that the dimensional changes when passing through the recalescence critical ranges are substantially less than in the case of the chromium-nickel-molybdenu'm steels and otherdie steel compositions used for making die blocks. This minimizing of dimensional changes is important because large changes in volume may, 'and commonly will act detrimentally, re;-

I .55 du'cing the useful life of the die. Hence the minimizing of such volume changes in the articles "provided by \this invention plays an important part in reducing the tendency to crack through internal strains, reduces the ,development and.

growth of thermalcracks, and is otherwise advantageous because when the highly heated forging stock the surface of the die no doubt promptly reaches a temperaturesubstantially that of the highly heated stock.

As evidencing in this connection the particularly desirable character of the dies provided by this invention, reference is made to' the accompanying graph which represents the recalescence volume changes,

used in the preferred "practice of the invention.

, tained from die steels suitable for die contacts the in mills per inch, of commonly 70 used die steels in comparison with that of a steel.

It will be noted from this that the volume change the volume change of the dies provided by" this invention is similarly shown to be not a summation, of the decreases in volume change of a plain carbon die steel produced by the alloying constituents used inthe practice of the invention.

Actual service use of die blocks madein accordance with this invention has amply demonstrated their superiority in service to other die blocks now in commercial use. For instance, blocks made in accordance with the invention have been placed in service in commercial hot forging production operations in different plants. The results obtained in their use were rated by the drop forgers themselves using as a standard the bestv production results which they had previously obthe particular article being produced. Such use of the blocks has shown that they successfully fulfill the various fabricating, heat treatment and service requirements. Among these tests the following tabulation shows the improvements attained through the invention.

According to the provisionsof the patent statutes, I have-explained the principle and mode of practicing my invention and have described what I now consider to represent its best embodiment. However, I desire to have it understood that, within the scope of the appended claims, the. invention maybe practiced otherwise than as specifically described.

. I claim? i 1. A hot work die block formed of steel comprising 0.4 to 0.7 per cent of carbon, 0.6 to 1.25 per cent of chromium, 0.6 to 1.0 per cent of manganese, 0.3 to 0.8 per. cent'of molybdenum, 0.08 to 0.2 per cent of vanadium, and the balance substantially iron.

2. A hot work die block formed of steel comprising 0.47 to 0.58 per cent of carbon, 0.85 to 1.15 per cent of chromium, 0.65' to 0.95 per cent of manganese, 0.4 to 0.6 per cent of molybdenum,

0.12 to 0.15 per cent oflvanadium, and the balance substantially iron.

'3. A'hot work die block formed of steel comprising 0.47 to 0.58 per cent of carbon, 20.85 to 1.15 per cent of chromium, 0.65 to 0.95 per cent of manganese, 0.4 to 0.6 per cent of molybdenum,

0.2 to'0.4 per cent of'silicon, 0.12 to 0.15 per cent John A. sUccoP. j

0.08 to' 0.2 per cent :of" vanadium, and the balance substantially iron. 

